A schoolboy returned home from school to collect his homework and found his mother's murdered body lying in a pool of blood.
It's thought the 11-year-old boy may have disturbed his mum's killer by ringing the doorbell.
He ran to a neighbour's house and banged on the door, shouting: 'Help my mum, help my mum.'
Amina Bibi, 43, from Forest Gate, East London, had suffered multiple stab wounds. Detectives said her murder may have a 'domestic element' to it.
They have so far arrested two men in connection with the killing.
Neighbour Jamal Omar, 41, said the boy came knocking on his door, screaming for help, moments after he let him into the block of flat via the intercom system.
The father-of-two said: "He came banging on my door, shouting for help. When I opened the door he said 'help my mum. I don't know what's wrong with her'.
"I went next door with him and saw Amina lying face down on the floor, covered in blood.
"I rang 999 and they told me to revive her but it was too late for me to do anything."
Detective Chief Inspector David Whellams, who is leading the investigation into Mrs Bibi's death, said: "As the son entered the flat he found the body of his mother.
"The family at the moment are bearing up as well as they can after this tragedy. The children are very upset and are being looked after by relatives."
Police believe that Amina's killer would have been covered in blood when he fled her East London flat.
Detective Chief Inspector Whellams said: "From intelligence we believe he has run to the back of the block of flats and away, as a blood trail in the flat suggests they left by the back of the property.
"We have a CCTV image that looks like he was carrying some sort of garment or cloth, like a jacket, towel or other clothing.
"We believe he would have been very noticeable and appeal for anyone who saw him to get in touch. At that time of day there were kids being dropped off at the school and there were people on their way to work as he ran down the road.
"People also need to look out for any discarded garments or items that may be covered in blood."
A 64-year-old man and a man in his forties have been arrested and are being questioned by police.